Hydrogen chloride is a reaction by-product of many chemical processes, which use chlorine gas. For example, in the manufacturing of polyurethane, the starting reactants are chlorine and carbon monoxide, which react to form phosgene (COCl2). Phosgene subsequently is reacted with amine (R—NH2) to form isocyanate (RNCO) and 2 moles of HCl. Polyurethane is a polymerisation product of isocyanate. Isocyanate does not contain chlorine and yet chlorine is consumed in the synthesis of phosgene. This creates an opportunity for chlorine recovery from the by-product HCl, especially if the latter cannot be sold. Further, there is an increased pressure to curtail transportation of liquid chlorine, which forces isocyanate producers to build their plants in the vicinity of chloralkali plants and necessitating close coupling of both plant operations. Similar opportunities exist in manufacturing of polycarbonates, titanium dioxide, chlorobenzene, chloromethanes, certain fluoro compounds, phosphonates, and the like.
Recovery of chlorine from by-product HCl has been a subject of many developments. Those could be principally divided into two groups: (i) catalytic oxidation and, (ii) electrolysis. In the first group commercial processes exist under trade names “Kel-Chlor”, “Shell-Chlor” or “MT-Chlor”. All of those processes are based on the Deacon reaction:4HCl+O2+cat.→2Cl2+2H2OThe catalytic processes are regarded as complicated because they require extensive separation to achieve product purity. Furthermore, since those processes are operated at temperature of 250° C. or more and involve highly corrosive reactants, the materials of construction must be able to resist severe corrosion. Such materials could be expensive. A few catalytic oxidation plants have been built, however they were plagued with numerous operating problems.
The electrolysis route comprises an anodic oxidation of chloride anions to chlorine paired with a cathodic reduction reaction. The most obvious cathodic process is reduction of H+ ions to H2. The only commercialised technology, offered by Uhde GmbH (Germany) is based on such process scheme. According to a reference list in Uhde's 1993 brochure “Chlorine and hydrogen from hydrochloric acid by electrolysis” some 14 HCl electrolysis plants were built worldwide. Following recent technology improvements, the key performance parameters for the Uhde process are as follows:
Operating current density:4-4.8kA/m2Cell voltage:1.92-2.06VPower consumption:1,500-1,600kWh/t Cl2Uhde process employs cells consisting of bipolar graphite electrodes, separated by PVC cloth diaphragms, all connected in series to form an electrolyzer. 22% and 21% HCl is fed separately to the anode and cathode compartments respectively. Following the electrolysis, the depleted (about 17%) HCl is passed to HCl gas absorption section, where its strength is re-adjusted to suit the electrolysis specifications.
A number of improvements to HCl electrolysis have been proposed over years in patents and other publications. Those improvements primarily aimed at intensification of the process (higher c.d.) and/or lowering power consumption. For example, U.S. Pat. No. 4,311,568 to Balko describes a process for electrolysis of HCl, which uses a solid polymer electrolyte membrane with an anode bonded to one side of the membrane, and the cathode bonded to the other side of the membrane. Both anode and cathode contain RuO2—IrO2 electrocatalyst. In this process H2 is still evolved at the cathode and the highest cited c.d. is 1,000 A/ft.2 (i.e. 10.75 kA/m2). The cell voltage data have only been disclosed for the 600 A/ft.2 (6.45 kA/m2) c.d. and with the optimised anode structure, it was about 1.8 V. That translates to power consumption of about 1360 kWh/t Cl2, assuming 100% anodic current efficiency. Balko has demonstrated a feasibility of operation at elevated (compared to Uhde process) current densities but he has not eliminated the parasitic reaction of O2 evolution at the anode. Traces of O2 in chlorine may lead to accelerated degradation of carbon-based components in the cell. U.S. Pat. No. 5,411,641 to Trainham, III et al. discloses a novel concept of HCl electrolysis, in which anhydrous HCl is directly fed to the anode compartment of the cell, while dilute HCl environment is maintained in the cathode compartment. It is argued that the anode will be exposed to a much higher chemical activity of HCl, which will translate to a lower cell voltage and higher operating c.d. The cell itself can incorporate a solid polymer electrolyte membrane such as Nafion with the electrodes bonded to each of the two membrane faces. Such structure is also referred to as Membrane Electrode Assembly (or MEA). Based on the cited example the current densities not higher than 7.8 kA/m2 were demonstrated and the cell voltages (and hence power consumption) were not much different than those described by Balko above. Still, Trainham, III concept appeared to have eliminated an HCl absorber from the overall HCl electrolysis plant scope. Finally, it also recognises that the process can be operated with oxygen reducing cathode to bring about further significant cell voltage reduction. However, no actual examples are given. On the other hand, U.S. Pat. No. 5,770,035 to Faita, is exclusively focused on HCl electrolysis with utilisation of the oxygen diffusion cathode. Thanks to the energetically more favourable cathodic reaction (i.e. electroreduction of O2) significant reduction of cell voltage can be achieved. For example, at c.d. of 3 kA/m2, the recorded cell voltage was about 1.2 V vs. 1.75 V in a reference experiment involving conventional H2-evolving cathode. The power consumption at 1.2 V can be calculated at about 910 kWh/t Cl2. While this new HCl electrolysis concept is quite attractive from the power consumption point of view, the operating c.d. is lower than that of a conventional Uhde process. Furthermore, gas diffusion cathodes are complicated in the design and not very reliable. Still, according to a paper by F. Federico (De Nora, S.p.A., Italy) presented at the De Nora Symposium (Venice, May 4-6, 1998) this process has been scaled-up to 2.5 m2 electrolyzers, which have been installed and operated, on a technology demonstration basis, at Bayer production site in Leverkusen, Germany.
U.S. Pat. No. 6,066,248 to Lyke et al. discloses yet another variation of the HCl electrolysis process in which anode comprising an electrocatalyst and ionomer is either bonded to the membrane separator (Nafion type) or to the anode backing material. In all cases the catalyst layers (anode and cathode) had a thickness of 2 μm. Lyke et al. have demonstrated operation of their cell with the following cathode reactions (best results cited):
Max. CurrentCellCatholyteDensity (C.D.)VoltagePressureCathode Reaction(kA/m2)(Volt)Temp ° C.(psig)H2 evolution201.8660-90atm.O2 reduction101.208060Fe(III) reduction101.2280atm.On the surface, regarding the process version with O2 reduction as the cathode reaction, Lyke et al. has tripled the c.d. of the Faita patent. However, they have demonstrated it only in a very small (5 cm2) laboratory cell for a brief period of time. It is obvious to those skilled in the art, that the scale-up of oxygen depolarised cathode is a formidable task—accordingly the De Nora technology (i.e. Faita patent) truly defines the present state of the art, as far as HCl electrolysis with oxygen diffusion cathode is concerned.
In their third electrolysis concept, where the cathode reaction is reduction of a multivalent metal chloride (e.g. Fe(III) chloride), Lyke et al. do not disclose it in the context of the overall process. However, in the earlier U.S. Pat. Nos. 2,468,766 and 2,666,024 Low discloses an HCl electrolysis process, in which Cu(II) or Fe(III) chloride is reduced at the cathode to Cu(I) or Fe(II) chloride, respectively. Given that the cathode process has now higher standard potential, e.g. +0.77 V for Fe(III)/Fe(II), than that of H2-evolving cathode (0.0 V), a corresponding decrease in cell voltage can be expected. The reduced metal chloride can be subsequently re-oxidised in an external reactor by contacting the spent catholyte solution with oxygen or air. In Low's inventions the HCl electrolyzer is cylindrical, un-separated and contains a solid graphite anode (annulus) and a porous graphite and a hollow-core cathode in the center. Due to the cylindrical cell geometry, the cathodic c.d. is about 70% higher than anodic c.d. HCl electrolyte containing Cu(II) or Fe(II) chloride is first passed by the anode, where Cl− ions are oxidised to Cl2 and then it is evacuated through the porous cathode to the oxidising section. A preference towards using Cu(II) chloride is stated and exemplified. The possibility of using a mixed Cu(II)—Fe(III)—HCl system is also mentioned without elaborating on potential benefits. In the Low's process concept, the electrolyte flow through the cell must be carefully optimised to: (i) allow disengagement of product Cl2, and (ii) to minimise back-diffusion of the reduced form of metal chloride (towards the anode). Likewise, the distance between the electrodes cannot be too close. Any portion of dissolved chlorine that comes into contact with the reduced metal chloride or the cathode constitutes a loss of c.e. In fact, under optimised conditions, Low has only achieved c.e.'s in the range of 81-85%. The highest cited cathodic c.d. was 509 A/sq. ft. (5.4 kA/m2) but the corresponding anodic c.d. was only 3.2 kA/m2. With a cell voltage of 2.69V and even allowing the upper limit of c.e. the calculated power consumption is 2,390 kWh/t Cl2. This value is significantly higher than that in a conventional Uhde process, indicating that despite the favourable thermodynamics resulting from employing cathodic reaction with a higher potential, the compromises made in the electrolyzer design (to maximise c.e.) had resulted in the overall un-impressive technical performance.
The idea of using cationic additives to facilitate oxidation of Fe(II) chloride with oxygen has been previously disclosed in GB Patent 1,365,093 to Kovacs who found that addition of cupric or cuprous ions and/or ammonium ion promotes oxidation of ferrous chloride by oxygen.
The concept of employing reducible metal chlorides for the cathodic reaction in the electrolysis of HCl is known in U.S. Pat. Nos. 3,635,804 and 3,799,860 to Gritzner et al. who have employed a filter-press type cell, with solid graphite electrodes separated by plastic cloth diaphragm. An external oxidiser for re-oxidation of spent catholyte is also disclosed. The cell had separate anolyte and catholyte circuits. Catholyte consisted of about 1.5M CuCl2 and 6M HCl. Spent catholyte had only a maximum 4.2% of original Cu(II) converted to Cu(I), with a significant decrease in c.e.—see example 34 in U.S. Pat. No. 3,799,860. Higher catholyte re-circulation rate kept c.e. high, however it also put a high hydraulic load on the oxidiser. Unfortunately, the highest c.d. employed by Gritzner et al. was only 1 A/in2 (1.6 kA/m2)—see examples and Claim 17 (in U.S. Pat. No. 3,799,860). Under condition of low catholyte (Cu(II)→Cu(I)) conversion, current efficiency in the low 90%'s and low cell voltage was achieved, as demonstrated in example 36 wherein the calculated power consumption at low c.d. of 1.6 kA/m2 was about 930 kWh/t Cl2. To put this value in context, the power consumption in the De Nora process, as per aforementioned paper by Federico is 900 kWh/t Cl2 but at a much higher c.d. of 3 kA/m2.
High surface area electrodes are known under the terms “3-dimensional electrodes” or “3D electrodes”. The 3D electrodes are characterised by an electroactive area, which is significantly higher than their projected area. The real surface area of 3D electrode can be calculated for regular structures such as uniform particle beds, woven fabrics, and the like. For irregular materials the real surface can be determined by methods known in the art e.g. BET adsorption method or mercury intrusion porometry.
Unlike planar or 2D electrodes, the 3D electrodes are also characterised by the finite thickness of the electroactive zone, wherein in 2D electrodes the electroactive zone is simply the plane of the conductive material, which is exposed to the electrolyte—and thus this plane has zero thickness. A good review on 3D electrodes is contained in Chapter 3 (Three Dimensional Electrodes) in “Electrochemistry for A Cleaner Environment”, edited by J. D. Genders and N. L. Weinberg, The Electrosynthesis Company Inc., E. Amherst, N.Y., 1992. On p. 52, the authors cite that the 3D electrodes have successfully been used for removal of low concentration of metal ions and organics from effluents prior to discharge. Subsequently, they teach that processing more concentrated solutions can introduce difficulties, such as plugging of the electrode porous structure with electrodeposited metal (page 80 and 86). In FIG. 3 (page 54) several typical configurations of cells, which employ 3D electrodes are shown. Apart from the electrode geometry, e.g. rectangular or cylindrical, one can distinguish two basic configurations: known as a “flow-by” configuration, in which electrolyte flow is normal to the current vector, and a “flow-through” configuration, in which the electrolyte flow is parallel to the current vector.
In summary, notwithstanding extensive development and certain progress made, there still is a need for an HCl electrolysis process, which offers both process intensification, i.e. high current density, and low power consumption.